Always init the BG opacity with the renderer

  Missed this in #11180. I forgot to init the BG opacity with the renderer on startup, because that matters when you have `"antialiasingMode": "cleartype",`.

  Repro json
  ```json
    {
        "commandline": "cmd.exe",
        "guid": "{0caa0dad-35be-5f56-a8ff-afceeeaa6101}",
        "hidden": false,
        "opacity": 35,
        "antialiasingMode": "cleartype",
        "padding": "0",
        "name": "Command Prompt"
    },
  ```

  * [x] Fixes #11315

src/cascadia/TerminalControl/ControlCore.cpp

@@ -274,10 +274,8 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             _updateAntiAliasingMode(_renderEngine.get());
 
             // GH#5098: Inform the engine of the opacity of the default text background.
-            if (_settings.UseAcrylic())
-            {
-                _renderEngine->SetDefaultTextBackgroundOpacity(::base::saturated_cast<float>(_settings.Opacity()));
-            }
+            // GH#11315: Always do this, even if they don't have acrylic on.
+            _renderEngine->SetDefaultTextBackgroundOpacity(::base::saturated_cast<float>(_settings.Opacity()));
 
             THROW_IF_FAILED(_renderEngine->Enable());
 

src/cascadia/TerminalControl/TermControl.cpp

@@ -459,7 +459,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             }
 
             // GH#5098: Inform the engine of the new opacity of the default text background.
-            _core.SetBackgroundOpacity(::base::saturated_cast<float>(appearance.Opacity()));
+            _core.SetBackgroundOpacity(appearance.Opacity());
         }
         else
         {